This disclosure relates generally to corona transfer systems for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to a method to improve the appearance of a ghosting print defect in such systems.
In xerographic direct transfer to paper systems, there are frequently ghosting issues associated with differential positive charge treatment due to positive charging transfer corona, for example. Paper at a sheet zone (SZ) or a document zone (DZ) partially blocks a photoreceptor of the system from the positive corona, while in an interdocument zone (IDZ) the photoreceptor is exposed to the complete positive current output of the transfer device. Current organic photoreceptors are designed to photo-discharge a single polarity of charge. In the systems under discussion, negative charging photoreceptors are combined with negative charging toner and discharge area development and positive transfer. This disclosure is not limited to any one type of polarity of charge. In typical systems, negative non-uniform residual charge can be minimized by photo discharge in an erase step post-transfer. However positive residual charge is not erased. These positive charges can only be discharged by a later negative charge step. If the positive charge is non-uniform and the negative charge step is not robust enough, then the non-uniform post-transfer charge may get trapped in one of the photoreceptor layers, and can result in non-uniform charge and development voltages and non-uniform image density in later photoreceptor cycles.
In particular, in the case of positive charge non-uniformities between the SZ and the IDZ, after aging of the photoreceptor with the same paper size for many thousands of prints, the non-uniformity will get “burned” into the photoreceptor. This will manifest in a number of ways. If the paper size is changed so that the previous interdocument zone becomes part of the image zone, then there will be a developed density difference causing banding between the previous IDZ and the SZ part of the image, the IDZ band being darker than the SZ. Also, if the IDZ is used for either voltage measurements or developed patches for electrostatic setup or process control, there will be a difference between the IDZ readings and the actual performance in the image area. In color systems this will cause color stability and color shift problems. A further problem occurs in the area of the photoreceptor corresponding to the edge of the paper. The transfer system is usually operated with a constant current device. The transition in impedance between the paper zone and IDZ causes a “ringing” or “edge leakage” in the residual charge at the edge of the paper resulting in a further banding defect known as “Paper Edge Ghosting” or “PEG”. Finally if the paper width is less than the photoreceptor width, the non-paper inboard or outboard region of the photoreceptor will have similar positive residual charge differences to the IDZ differences and will result in darkened regions of subsequent images if the paper size is changed to cover parts of the previous inboard or outboard non-paper regions.